Retransmission control in a conventional OFDM system will now be described taking a case where a transmitting-side apparatus performs retransmission to a receiving-side apparatus as an example. First, the transmitting-side apparatus performs IFFT (inverse Fourier transform) processing using an information signal, etc., for the receiving-side apparatus, and thereby generates an OFDM signal in which that information signal is superimposed on each subcarrier. The transmitting-side apparatus then generates a burst unit signal by performing predetermined transmission processing on the generated OFDM signal, and transmits the generated burst unit signal to the receiving-side apparatus.
The receiving-side apparatus extracts the signal superimposed on each subcarrier by performing FFT processing (Fourier transform processing) on a received signal for the above-described burst unit signal. The receiving-side apparatus then generates a demodulated signal by performing demodulation processing on the extracted signal, and performs error detection processing on the generated demodulated signal. If an error has occurred in the generated demodulated signal, the receiving-side apparatus transmits a burst unit signal containing a signal requesting retransmission of the above-mentioned burst unit signal (hereinafter referred to as “retransmission request signal”) to the transmitting-side apparatus. A burst unit signal in the receiving-side apparatus is generated by means of the same kind of processing as in the transmitting-side apparatus described above.
On receiving a burst unit signal containing a retransmission request signal from the receiving-side apparatus, the transmitting-side apparatus generates and transmits again (retransmits) the above-described kind of burst unit signal using the same information signal as the above-described information signal. By this means, the receiving-side apparatus can generate a demodulated signal with improved error rate characteristics.
The same kind of retransmission control as in the OFDM method, as described above, is also performed in the OFDM-CDMA method, which combines the OFDM method and CDMA method. Retransmission control in the OFDM-CDMA method is similar to retransmission control in the OFDM method except for the following point. Namely, in the OFDM-CDMA method, the transmitting-side apparatus generates an OFDM signal by performing IFFT processing using an information signal that has undergone modulation processing and spreading processing, and the receiving-side apparatus generates a demodulated signal by performing despreading processing and demodulation processing on a signal extracted by means of IFFT processing.
Meanwhile, communication has been investigated in which multicasting is applied to the OFDM method and OFDM-CDMA method (hereinafter referred to simply as “OFDM method”). Multicasting is communication whereby a transmitting-side apparatus transmits a specific (identical) burst unit signal to a plurality of receiving-side apparatuses (that is, communication whereby a plurality of receiving-side apparatuses receive a burst unit signal transmitted by the same transmitting-side apparatus). Retransmission control in an OFDM method to which this kind of multicasting is applied is similar to retransmission control in the OFDM method described above, except for the following point.
Namely, the transmitting-side apparatus transmits the same burst unit signal to a plurality of receiving-side apparatuses, and if an error occurs in a generated demodulated signal, the plurality of receiving-side apparatuses transmit a burst unit signal containing a retransmission request signal to the transmitting-side apparatus. On receiving such a burst unit signal from any of the plurality of receiving-side apparatuses, the transmitting-side apparatus retransmits the above-described burst unit signal to the plurality of receiving-side apparatuses. By this means, the plurality of receiving-side apparatuses can generate a demodulated signal with improved error rate characteristics.
However, there is a problem with the above-described conventional OFDM method, as follows. Namely, when an error occurs in a demodulated signal, the transmitting-side apparatus performs retransmission to a receiving-side apparatus in response to a retransmission request issued by the receiving-side apparatus to the transmitting-side apparatus. By this means, a receiving-side apparatus can improve demodulated signal error rate characteristics.
However, in an OFDM method to which multicasting is applied, since a specific burst unit signal transmitted by a transmitting-side apparatus is received by a plurality of receiving-side apparatuses, there are cases where a plurality of receiving-side apparatuses in which an error has occurred in the received burst unit signal transmit a burst unit signal containing a retransmission request signal simultaneously in the same frame. This situation will be described in detail with reference to FIG. 1. FIG. 1 is a schematic diagram showing the frame format used in the conventional OFDM method.
The transmitting-side apparatus transmits specific burst unit signals (that is, down burst 1 and down burst 2) to a plurality of receiving-side apparatuses. Of this plurality of receiving-side apparatuses, a receiving-side apparatus that detects an error in the demodulated signal for the above-mentioned burst unit signals transmits a burst unit signal containing a retransmission request signal (for example, retransmission request burst 1) to the transmitting-side apparatus. If there are a plurality of receiving-side apparatuses that have detected an error in the demodulated signal, each of such receiving-side apparatuses (in FIG. 1, four receiving-side apparatuses) transmits one burst unit signal.
Therefore, the greater the number of receiving-side apparatuses that detect an error in the demodulated signal, the greater is the time occupied by these burst unit signals containing a retransmission request signal. As these burst unit signals containing a retransmission request signal are not directly involved in information signal transmission, information signal transmission efficiency falls in proportion to the amount of time occupied by burst unit signals containing a retransmission request signal in the same frame. A fall in information signal transmission efficiency can be prevented by arranging for a receiving-side apparatus that detects an error in a demodulated signal not to transmit a burst unit signal containing a retransmission request signal. In this case, however, demodulated signal error rate characteristics in a receiving-side apparatus will deteriorate.
As described above, a problem with an OFDM method to which conventional multicasting is applied is that it is difficult to achieve both an improvement in information signal transmission efficiency and an improvement in demodulated signal error rate characteristics, due to the fact that there are cases where a plurality of receiving-side apparatuses that have detected an error in a received burst unit signal transmit burst unit signals containing a retransmission request signal in the same frame.